LITTLE ROCK (AP) — Democratic U.S. Sen. Mark Pryor on Saturday said he’s opposed to taking military action against Syria, splitting with the White House and his Republican challenger as a vote on the president’s request to intervene heads to a skeptical Congress.

The two-term senator from Arkansas said he cannot support the resolution that’s expected to go before lawmakers in the next week to intervene against Syria, which U.S. officials say killed more than 1,400 civilians in a chemical weapons attack last month.

“I have said, before any military action in Syria is taken, the administration must prove a compelling national security interest, clearly define a mission that has a definitive end-state, and then build a true coalition of allies that would actively participate in any action we take,” Pryor said in a statement released by his office. “Based on the information presented to me and the evidence I have gathered, I do not believe these criteria have been met, and I cannot support military action against Syria at this time.”

Pryor told The Associated Press he had talked with White House chief of staff Denis McDonough earlier this week and Friday night about Syria, and had made his decision Friday night. While Pryor’s statement appeared to leave open the possibility that he’d change his mind, the senator later said that was very unlikely.

“Unless there’s some new information or some new development or circumstance, I just don’t see me changing my vote at this point,” he said.

Pryor’s stance puts him not only at odds with the Obama administration, but also Republican rival and U.S. Rep. Tom Cotton. Cotton has emerged as one of the most vocal advocates for taking action against Syria and has warned that not approving the request would weaken the presidency in the eyes of other nations.

Pryor’s decision complicates what already appeared to be an uphill battle in convincing a war-weary Congress to back the resolution. A survey by the AP shows House members staking out positions are either opposed to or leaning against President Barack Obama’s plan for a U.S. military strike against Syria by more than a 6-1 margin. The Senate is more evenly divided ahead of its vote next week.

Still, the situation is very fluid. Nearly half of the 433-member House and a third of the 100-member Senate remain undecided.

Arkansas’ other three Republican congressmen and Republican Sen. John Boozman have all said they’re likely to oppose the request.

The Syria vote has injected foreign policy into a Senate race that many had expected to hinge on domestic issues like health care. Republicans view Pryor as the most vulnerable Senate incumbent running next year, and they believe they’re poised to unseat Pryor by tying the lawmaker to Obama and his signature health care overhaul.

Pryor said the Senate race and Cotton’s support for military action didn’t factor into his decision to oppose intervention.

“This issue is way beyond politics,” Pryor said.

As he campaigned at a Texarkana picnic on Labor Day, Pryor encountered several voters who urged the Democratic lawmaker to oppose the request for military action. Pryor on Saturday said the feedback he’s heard from constituents has been “overwhelmingly” against the resolution.

“When you lay out that test, I just feel like I haven’t heard a compelling case made by the president and think most people in Arkansas haven’t heard that,” Pryor said. “I think I’m on the same page as Arkansas here.”

Cotton, an Army veteran who served in Iraq and Afghanistan, had advocated action against Syria before Obama announced he would seek congressional approval for a strike. He’s also urged colleagues to back the president’s request, co-authoring an op-ed in the Washington Post last week with Republican Rep. Mike Pompeo of Kansas.

“What I hope to do is encourage not just my colleagues to support action but also encourage the president to take effective and decisive action,” Cotton told the AP on Monday.

I'm sure he also said if the president had been republican he would have voted for whatever the president thought best regardless of the impact on US civilians. After all, he did do that with Bush every single time.